Since thermal power plants and garbage incineration plants have low restriction (lenient regulations) for installation site, the thermal power plants and the garbage incineration plants are constructed at many locations all over the world. Moreover, since a large amount of carbon dioxide is exhausted from these plants, the emission reduction of the carbon dioxide calls for urgent attention in view of the recent interest to global warming issue and the background of tightening of regulation for the global warming.
In order to reduce the amount of emission of the carbon dioxide, the high efficiency of the plants is attempted while a method and system for absorbing the carbon dioxide exhausted. For example, it is taught in a conventional technique that calcium hydroxide (Ca(OH)2) and ammonia (NH3) are reacted one another to produce ammonium hydroxide (NH4OH), which is introduced with seawater into a predetermined treatment equipment so that the exhaust gas containing the carbon dioxide is contacted with thus obtained solution containing the ammonium hydroxide and the seawater and thus the carbon dioxide is absorbed in the solution.
In this method, however, the calcium hydroxide is made through the hydrolysis of calcium oxide (CaO) and the calcium oxide is made through the thermolysis of calcium carbonate (CaCO3). In the thermolysis of calcium carbonate, carbon dioxide is also produced with the calcium oxide at equimolar concentration. In the aforementioned method, therefore, when the ammonium hydroxide as the raw material to be used for the absorption of the carbon dioxide is produced, the carbon dioxide is also produced at equimolar concentration so that even though the carbon dioxide in the exhausted gas is absorbed with the ammonium hydroxide, additional carbon dioxide is produced again at the same amount as absorbed carbon dioxide. As a result, the problem of the reduction of the emission amount of the carbon dioxide cannot be realized.
Moreover, a plurality of by-products are produced at the absorption of the carbon dioxide, but the conventional technique does not teach the treatment for the by-products. Therefore, the aforementioned conventional technique cannot be applied for the absorption of the carbon dioxide exhausted from the practical plants.